A-1 : In Oracle RAC, split-brain is the scenario when one or more nodes updates to the database files w/o considering the integrity with other nodes. so in that scenario there is high possiblity of compromissing of database integrity and introducing the corruption to the database.
Q-2: What is the role of voting disk/file in RAC?
A-2: In Oracle RAC, voting disk file is used to determine the state of each nodes in the cluster. Each node should write heartbeat to the voting disk in predetermine interval i.e. 1 sec, so other nodes in the the cluster know that the node is alive. If node could not register the heartbeat to voting disk in stipulated time frame then it should be fence out from cluster to avoid split-brain scenario, which might introduce corruption to the database. Oracle Cluster Synchronization Service Daemon(OCSSD) is responsible to maintain synchronization of the cluster using voting disk.